Cultured hippocampal neurons from fetal trisomy 16 mouse (Ts16), a model for human trisomy 21 (Down syndrome, DS), showed a decreased voltage-dependent sodium current measured by patch-clamping, and fewer Na channels assessed by binding of radiolabeled saxitoxin, relative to feta diploid neurons. mRNAs encoding the a and b1 subunits of Na channels were unchanged, indicating reduced post-transcriptional regulation of channel assembly. High-voltage-activated calcium currents were larger in Ts16 than diploid neurons, consistent with higher binding of an L-type calcium ligand. However, N-methyl-D-aspartate (NMDA)-evoked currents in hippocampal Ts16 neurons did not differ from currents in diploid neurons, and ultrastructural morphology of fetal Ts16 and hippocampal and dorsal root ganglion neurons was normal. If changes in active Ca and Na currents also occur in Down syndrome brain, they may contribute to mental retardation. Hypotonia is a feature of Down syndrome. Two currents that affect repolarization of action potential were abnormal in cultured Ts16 fetal tongue muscle cells, consistent with hypotonia. Chloride conductance was increased and potassium conductance was decreased. A PCR amplification assay using polymorphic simple-sequence repeats was developed to identify partially trisomic mice from colony of TS65DN mice, in which a region of chromosome 16 is duplicated that is syntenic with the duplicated region in trisomy 21 leading to mental retardation. Cerebral cortical astroglia from the Ts16 mice produce neuronal cholinergic deficits in cell culture.